1. Field of the Invention
This invention relates to swim fins, more particularly to high-performance diving fins and especially to fins for shore diving and amphibious activities.
2. Previous Art
As even the best swimmer knows, the human leg and foot are almost useless for propulsion in water. While the foot is an elegant adaptation for walking, it is blunt, stiff and naturally oriented at a right angle to the lower leg. By kicking harder, a swimmer actually wastes effort and increases drag. To recapture some of the aquatic performance that was sacrificed eons ago as the price of terrestrial evolution, swimmers wear fins. However, fins are clumsy on land and their propulsive effectiveness is limited by the configuration of the human knee and ankle, which make it difficult to angle the fin properly.
The angle of a fin is important because the fin propels the swimmer by imparting momentum to a mass of water, such that the swimmer gains equal momentum--and is moved--in the opposite direction. Generally, as the swimmer kicks, each fin reciprocates, its upper and lower surfaces alternately pressing against the water. Such reciprocation alone would only stir the water. However, because the fin bends, when a fin surface is pressing into the water, it is usually also facing at least partially toward the rear. Thus, during a complete stroke, the fin imparts a net rearward momentum to the water and the swimmer is correspondingly propelled forward.
The kicking movement employed by swimmers and divers can be described with reference to a starting position in which the leg is straight at the knee and in line with the long axis of the spine. First, the quadriceps relaxes and the hip flexor begins to contract. The hip flexor contraction moves or flexes the upper leg forward at the hip. The relaxation of the quadriceps allows the knee to bend as the upper leg moves forward. Once the knee has moved forward, contraction of the quadriceps straightens the leg at the knee, causing the foot and the fin to move forward. The bending at the knee orients the dorsal surface of the foot and therefore the dorsal surface of the fin blade at an angle, which imparts a rearward momentum to the water, propelling the swimmer forward.
Swimmers and divers employ two common kicking movements: the flutter kick and the undulating (or butterfly, or dolphin) kick. In the flutter kick, one leg flexes while the other leg extends. With this kicking movement, the weaker direction of movement (upper leg extension) will limit the muscles involved in the stronger direction of movement (upper leg flexion followed by lower leg quadriceps extension) to the levels of force established by the extension muscles, because the kicking movement must remain balanced. Much drag is created as the legs work against each other. In the undulating kick, both legs move in the same direction. Flexion at the hip is followed by contraction of the quadriceps to straighten the lower leg at the knee. Hamstring and gluteal contraction follow to extend the legs back to the starting position. The undulating kick usually involves greater angles of movement than the flutter kick.
Because conventional fins extend in the general direction of the toes, the fin angle depends on the angle of extension of the foot, which is limited to the range of motion of the ankle joint. During the return stroke, the swimmer can extend the foot and point the toes in order to angle the plantar (foot bottom) surface of the fin aft for fairly efficient propulsion. All too soon, however, the upper leg reaches the limit of its rearward motion and it is time to bring the leg forward and bend the knee in preparation for the next kick stroke. As the knee is bent and brought forward, the thigh, calf and heel create drag.
During the kick stroke, the powerful quadriceps muscle can be applied. However, most people's feet will not extend (point downward) far enough to place the fin at a propulsive angle during the whole kick stroke. This problem is only partly remedied by the flexibility of the fin. While flexibility can enable the distal portions of the fin surface to assume a propulsive angle after kicking force is applied, this flexing occurs only after significant energy has been wasted and leaves the proximal portion of the fin at an angle which produces much drag and little propulsion.
Even more seriously, the ligaments of the human ankle are too weak to withstand the full kicking force of the quadriceps driving a large fin in water. Swimmers who deliberately kick with full force while wearing a large fin will incur severe damage to their ankle ligaments. There are estimates that the quadriceps can generate up to three times the kicking force that the ankle ligaments can safely deliver to a large fin. Thus, a need exists to more quickly and more efficiently orient the fin at the ideal propulsive angle, especially during the kick stroke. A need also exists to couple the powerful kick of the quadriceps to a large diving fin without overloading the ligaments of the ankle.
Another difficulty with fins is encountered at the beach. Fins are clumsy for walking on any surface, even more so in shallow water, and especially in currents. The muscles that elevate the human foot are very weak and are no match for the power of the sea against a long SCUBA fin. Surf pounding against a long fin will easily trip and upset a wading diver. Although the diver may carry the fins into calm water and then put them on, the fins must be worn when entering surf, for a finless diver is a helpless diver and will be driven back onto the beach by even moderate waves. The results may be merely frustrating or they may be far more serious, depending on how urgently the diver needs to move along.
Divers would benefit were they able to walk, or even run, into shallow surf with their fins attached, but positioned so as not to interfere with foot placement. Particularly advantageous would be a way of attaching a fin to the lower leg and orienting the fin upward, adjacent the lower leg, and then quickly deploying the fin in a propulsive position once the water is deep enough for the fins to function. What is especially needed is a swim fin which allows a diver to wade against waves or surf in shallow water, which can be deployed quickly in the transition from wading to swimming, which is oriented at a propulsive angle during the kick stroke as well as the return stroke, and which enables the diver to kick powerfully yet comfortably and safely. Additionally, a swimmer whose foot is injured, malformed, or for any other reason cannot bear the forces that attend the use of a conventional diving fin would benefit from such an innovation.